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Disinfection Byproduct Control

Surface waters contain both pathogens and low levels of organic material (for example, decaying plant matter and algae). To insure that distributed water is safe to drink, the USEPA mandates that drinking water plants achieve specified levels of pathogen inactivation. One tool for insuring pathogen inactivation is to add sufficient disinfectant to maintain a residual concentration at customers’ taps.

In addition to inactivating pathogens, disinfectants react with the organic matter present to form disinfection byproducts (DBPs). Epidemiological studies have identified two classes of DBPs, trihalomethanes (TTHMs) and haloacetic acids (HAA-5s) as being human carcinogens.  To reduce the public’s exposure to these carcinogens, early this decade the USEPA implemented the following rules:

  • The TOC rule mandates that drinking water plants achieve specific levels of TOC removal based on the TOC content and alkalinity of their raw waters.
  • The DBP Rule mandates that drinking water plants produce water containing less than specific levels of TTHMs and HAA-5s.

Achieving TOC compliance primarily involves identifying the compliance criterion that best conforms to the characteristics of the raw water being processed, and using an appropriate dosage of the correct coagulant. Since algae are not typically readily removed by coagulation, when algae represent a significant fraction of a plant’s raw TOC, standard  algae removal methods are sometimes implemented, including the use of algaecides and ortho-phosphate (nutrient) control by precipitation with coagulants such as alum.

Although the criteria for achieving DBP Rule compliance are straightforward, achieving compliance itself is more complex and depends on a variety of issues, including: the concentration of TOC present at the point of disinfection, the specific disinfectant strategy used, specific plant design issues, the water temperature and pH, and the detention time of the distribution system.

When plant design allows, the USEPA recommends removing TOC via coagulation and sedimentation prior to the addition of disinfectant.  This is a simple and particularly effective method for reducing DBP formation since it reduces both the amount of organic material present at the disinfection point, as well as reducing the disinfectant demand.  Since chloramines generate lower quantities of DBPs than Cl2, the use of chloramines as distribution system disinfectants has steadily increased since implementation of the DBP Rule.

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